PROJECT SUMMARY/ABSTRACT Innate immunity plays important roles as first line defense and primer for adaptive immunity to protect against infection, and its excessive prolonged activation promotes chronic inflammatory diseases. While the main molecular players and signaling pathways involved in innate immunity have been identified, more research is needed to understand how signaling among multiple tissues triggers innate immune responses at the organismal level. Since studying multi-tissue innate immune responses remains challenging in vertebrate systems, we address this question in a simple invertebrate model. Drosophila melanogaster has been key in the discovery of innate immunity, and it is likewise expected to be an excellent model to understand molecular mechanisms that drive more complex, multi-tissue innate immune responses. Specifically, we propose to investigate a new model of an innate immune response in adult Drosophila, which involves the combination of a reservoir of immune cells (hemocytes), respiratory epithelium, and domains of the anatomically colocalizing immune tissue of the fat body. In this model, we focus on the expression of Drosocin as a readout, which promotes survival after bacterial infection. We find that hemocytes, and specifically their signaling by the NFkB- related Imd pathway, are required for the induction of Drosocin expression in the respiratory epithelium and locally restricted domains of the fat body. However, while Imd signaling in hemocytes is required, it is not sufficient to trigger the Drosocin response. We hypothesize that immune cells act as sentinels of bacterial infection that relay a (so far unidentified) signal to the respiratory epithelium and fat body, which in response upregulate Drosocin. Drosocin has, at endogenous expression levels, anti-bacterial function and promotes animal survival after bacterial infection. We propose to (1) identify hemocyte signal/s that trigger the Drosocin response in other tissues, and (2) identify the signaling pathways within tissues that relay the Drosocin response. This work is significant because insights from this Drosophila model are expected to increase our understanding of the molecular mechanisms that drive multi-tissue innate immune responses in a variety of organisms across phyla, thereby extending the concept of `inter-organ/-tissue communication' to innate immunity. New mechanistic principles identified in this model are expected to inform vertebrate research and inspire therapeutic approaches that curb or enhance multi-tissue innate immune responses, which could be tailored toward a variety of medical needs and conditions.